Geologic History of Popes Creek, Virginia

Surficial geologic maps of small, meso-tidal estuaries and their watersheds can be interpreted as scale independent analogs to understand the function of larger estuarine systems. Popes Creek, a minor tributary to the Virginia side of the Potomac River estuary, is an effective trap for sediment and nutrients derived from a variety of forested and farmed watersheds that drain broad, low-relief terraces and dissected upland slopes. Situated in Westmoreland County, Virginia, the watershed of the Popes Creek estuary includes drainage from the George Washington Birthplace National Monument; long-term records of land use document events affecting entrainment, transport, and movement of sediment from the inception of Colonial agriculture to the present. This estuary is a particularly effective trap for watershed-derived sediment because its mouth is plugged by a flood-tide delta formed by rising sea level and long-shore movement of Potomac River sediment stripped from nearby eroding bluffs and beaches.

We have developed a surficial geologic map of the Popes Creek watershed that documents a system of weathering, erosion, slope deposition, and fluvial to estuarine terrace deposition that has been moving, storing, weathering, and reworking sediments since the end of the Pliocene (~1.6 Ma). Sediment in the estuary is derived from three primary sources: cleared land in the watershed, the Potomac River, and shoreline erosion along Popes Creek. Present geomorphic processes Pare generating most new sediment from sheet wash on cultivated fields, slope failure driven by discharge of springs in headward eroding gullies, and sheet wash on forested, steep slopes. Much of the sediment is stored in ravines and on flood plains of the larger tributaries; we have observed as much as 2 meters of agriculturally derived ravine fill covering logs and stumps that date from the middle of the 17 th century. On broad, low gradient flood plains, modern sediment is being transported in a random but peristaltic cadence of storage and erosion; braided channel, alluvial surfaces alternate with deeply gullied reaches. Aliquots of sediment are added from slope deposits and alluvial fans along the valley margins. Entire flood plains are marked by a series of breached and current beaver dams and ponds. Several old millponds also interrupt the flow of modern sediment. Presently, minimal sediment reaches the distal ends of tributary deltas, which are accumulating freshwater peat. Much of the modern, watershed-derived sediment appears to be stored in the fluvial part of the system.

Historical evidence about Popes Creek estuary suggests that navigation has been limited to shallow-draft vessels since Colonial times; today the estuary is, for the most part, 1 meter or less deep and has a tide range of 0.3 to 0.4 meter. Initial coring in the flood-tide delta has penetrated almost 15 meters of mollusk-bearing sediments overlying fresh water peat and fluvial sand dated at 6,280 years before present. The Popes Creek estuary continues to shoal as rising sea level drives the products of coastal erosion from the Potomac River a kilometer or more inland. Erosion of the bluffs along the Popes Creek estuary is a third significant source of sediment that has filled the tidal sediment trap. Our studies of the Popes Creek estuary provide a baseline for comparison with other Coastal Plain watersheds that are being more intensively cultivated and modified.